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Pollution by Mud of Great Barrier Reef Coastal Waters Eric Wolanski and Simon Spagnol

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Pollution by Mud of Great Barrier Reef Coastal Waters Eric Wolanski and Simon Spagnol Journal ofCoastal Research 1151-1156 West Palm Beach, Florida Fall 2000 Pollution by Mud of Great Barrier Reef Coastal Waters Eric Wolanski and Simon Spagnol Australian Institute of Marine Science PMB No.3 Townsville Me, Qld. 4810, Australia ABSTRACT _ WOLANSKI, E. and SPAGNOL, S., 2000. Pollution by mud of Great Barrier Reef coastal waters. Journal ofCoastal Research, 16(4), 1151-1156. West Palm Beach (Florida), ISSN 0749-0208. Oceanographic data from the coastal waters of the Great Barrier Reef near Cairns were obtained for two months in the 1997 dry season when river runoff was negligible. A muddy coastal zone was found with suspended sediment concentration reaching 1000 mg/L during trade winds. These high turbidity events are very stressful conditionsfor coral reefs and were caused by resuspension of sediment by wind waves. In calm weather a nepheloidlayer prevailed, extending about 30 km offshore and carrying coastal mud toward offshore coral reefs. In this layer, muddy marine snow with floes exceeding 1000 urn in diameter was found inshore, while only small floes < 100 urn were found offshore. Near-surface visibilitymay have decreased by 50% in the last 70 years. High turbidity appears to be due to man and ultimately threatens the main bodyof the Great Barrier Reef. ADDITIONAL INDEX WORDS: Mud, turbidity, siltation, resuspension, nepheloid layer, Great Barrier Reef. INTRODUCTION METHODS Most of our knowledge of the historical, physical properties For six weeks, in August and September 1997, six ocean­ of the waters ofthe Great Barrier reef (GBR) is derived from ographic moorings were maintained in a cross-shore transect the 1928-1929 British Museum Expedition. This pioneering at sites 1-6 shown in Figure 1 in depth varying from 3 m study was mostly carried out near Low Isles (16°23'S, inshore to 23 m offshore. Water depth and equipment de­ 145°34'E; see Figure 1 for a location map). MOORHOUSE ployed are summarised in Table 1. At sites 1-5 the moorings (1933) and ORR (1933 a and b) reported clear water (mean included Inter-Ocean model S4 current meters at about half­ visibility =11 m) during the South East trade wind season depth, a Dataflow salinometer and a total of 19 nephelome­ ters spread across the water column, the lowest one being from about April to October. Human impacts at that time located 0.5 m from the bottom. The units averaged data over were negligible. Since that time most of the forest and nat­ 1 min and recorded these data at 5 min intervals. ural vegetation that covered the land have been cleared and At site 6 near Pixie Reef an Inter-Ocean model S4 current much of it is now intensively farmed with usually no protec­ meter was deployed 6m from the bottom, and a Dataflow sa­ tive cover against soil erosion. This resulted in a large in­ linometer at 17m off the bottom in 25 m depth. During moor­ crease in mud discharged by rivers on the coast of the Great ing deployment and recovery, and also in December 1997 Barrier Reef (ANON, 1993; WOLANSKI, 1994, LARCOMBE et when dry weather conditions prevailed, vertical profiles of al., 1996, WACHENFELD et al., 1997). Historical photographs salinity, temperature and suspended solid concentration and navigation maps of the Cairns waterfront (see location (SSC) were obtained using a CTD equipped with an Analite in Figure 1) suggest that what was largely a sandy coast is backscatterance nephelometer. The nephelometers were cal­ now buried under 1.5 m of mud (WOLANSKI and DUKE,2000). ibrated in-situ for suspended solid concentration. In-situ pho­ The increased muddiness of the coast has been reported also tographs of the suspended matter were obtained using the at other locations along the coast of the Great Barrier Reef method of AYUKAI and WOLANSKI (1997). following European settlement. For instance, WOLANSKI Suspended solids were collected at 2.5 m from the bottom (1994) reported an invasion by mud of a sandy beach flat at every 2 days at site 5 using a McLane automated suspended Townsville (19.3°S). Large areas of mangrove forests that act­ sediment trap and the samples were was analyzed in the lab­ ed as efficient sediment traps (FURUKAWA et al., 1997), have oratory following the techniques described by WOLANSKI et also been removed by farming. Many estuaries essentially ai. (1999). have become drains bringing all the eroded, fine sediment Wind and rainfall data were obtained from meteorological directly to the sea. In view of this man-made increased mud­ stations at Green Island and Cairns. diness of coastal waters, oceanographic data were collected to assess the offshore extent of the dispersion of the mud and RESULTS the potential threat it poses to the Great Barrier Reef. Southeast trade winds peaking at 15 m S-1 prevailed throughout the study period, occasionally interrupted by 99050 received 17 May 1999; accepted in revision 22 December 1999. short periods of calm weather (Figure 2). The tides were 1152 Wolanski and Spagnol Table 1. Mooring details. ,~. '·'i~'·\,··f Depth of ~ Site Water current Depth of Low Isles ;~BallRf. number depth (m) meter (m) nephelometer (rn) 1 2 N/A 0.25,1 2 4 2.5 1,2,3,3.5 Pixie Rf. 3 6 4.5 3,4,5,5.5 6 '~ 4 11 6 6, 8, 10, 10.5 -f 5 18 12 14, 17, 17.5 6 25 19 N/A made of mucus, plankton detritus as well as other biological detritus. Offshore the suspended solids were present as in­ organic mud floes < 60 J.Lm in diameter. The spectrum of sea level (not shown) revealed that the semi-diurnal tides contained most of the energy. On the other hand the alongshore currents and wind showed negligible en­ ergy at diurnal and semi-diurnal tidal frequencies; most of the energy was contained at low-frequencies with period> 3 days with no apparent peak in the spectra. The wind showed a weak peak at diurnal frequency, an indication of a sea­ Figure 1. General location map showing the mooring sites. breeze effect. The spectra of SSC at stations 1 and 2 were weakly energetic at the diurnal frequency, possibly an effect of the sea-breeze, but not at all at the dominant tidal semi­ mostly semi-diurnal with a strong diurnal inequality, peak­ diurnal frequency. Highest energy in the spectra was at ing at about 3 m peak-to-trough at spring tides and 0.5 m at about 10 days period. This period is the same for the peak in neap tides (Figure 2). Freshwater runoff and associated riv­ the wind and current spectra, it is also the maximum period erine sediment inflow was negligible during the study be­ for which the spectrum can be reliably calculated from this cause it was carried out in the dry season. time series. The currents were mostly alongshore, alternating north­ Alongshore wind offshore (at Green Island) and currents ward and southward at periods of several days, the tidal var­ were significantly correlated with a lag increasing with dis­ iations were small and the low-frequency currents were dom­ tance offshore, the wind preceding the current (R = 0.8 at 2 inant (Figure 2). The currents were southward with speed of h lag at site 2; R = 0.6 at 8 h lag for site 3 and R =0.8 at 16 about 0.2 m S-l during calm weather and northward with a h lag at sites 5 and 6). speed of about 0.4 m S-l during strong trade winds and fluc­ The alongshore currents and the SSC were also signifi­ tuated very little with the tides. cantly correlated, the current preceding SSC, R = 0,.7 at 2 The suspended solid concentration (SSC) fluctuated widely hr lag at site 2, R = 0.45 at 6 hr lag at site 3, R = 0.5 at 13 in coastal waters (Figure 2). Maximum SSC was about 1,000 h lag at site 4. A negative and significant correlation between mg 1-\ at which time the visibility as reported by divers at wind and current occurred at site 5 located further offshore, site 1 was zero (it was pitch black at 2 m depth and the divers R = -0.5 at zero lag. No correlation (lRI < 0.2) between cur­ could not see their hand against the face mask). The SSC was rents and SSC prevailed at site 5. smaller further offshore. The SSC did not vary with the tides. For sites 1 and 2 a significant correlation also existed be­ Inshore, high SSC occurred frequently, but only during tween alongshore wind velocity and SSC; at site 1 R = 0.7 strong wind; at these times SSC was uniform throughout the at zero lag and 0.9 at 14 h lag; at site 2 R = 0.7 at 2 h lag. water column. Offshore, high SSC events (>50 mg 1-1 peak­ For site 3, no strong correlation was apparent (R < 0.4). At ing in one event at 200 mg 1-1) occurred occasionally. At these site 4 the correlation was negative and significant, R = -0.45 times the high SSC events were limited to near the bottom at zero lag. At site 5 no correlation was found between the (Figure 3a). During strong winds, suspended solids consisted wind velocity and the SSC (lRI < 0.2). of fine terrigenous mud forming floes typically 30-60 J.Lm in The SSC fluctuated coherently with distance offshore be­ diameter (not shown). tween sites 1-4. Between sites 1 and 2, R = 0.75 at 0.4 h lag; In calm weather a near-bottom nepheloid layer was ob­ between sites 1 and 3, R = 0.73 at 2.3 h lag and between served apparently cascading down the slope (Figure 3b).
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